187. Arrange these soluble cations (𝑁𝑎, 𝐶𝑎,𝐾, 𝑀𝑔) in ascending order of abundance of soluble
cations found in saline water bodies:
A. 𝑁𝑎 > 𝑀𝑔 > 𝐶𝑎 > 𝐾
B. 𝑀𝑔 > 𝑁𝑎 > 𝐶𝑎 > 𝐾
C. 𝐾 > 𝑀𝑔 > 𝐶𝑎 > 𝑁𝑎
D. 𝑁𝑎 > 𝑀𝑔 > 𝐾 > 𝐶a

Explanation:

In saline water bodies, the most common dissolved cations are sodium (Na⁺), magnesium (Mg²⁺), calcium (Ca²⁺), and potassium (K⁺). These cations contribute to the ionic composition of seawater.

The general order of abundance in seawater is as follows:

1. Sodium (Na⁺) – The most abundant cation in seawater, accounting for around 85% of the total dissolved cations.

2. Magnesium (Mg²⁺) – Second in abundance after sodium, comprising about 10% of the dissolved cations.

3. Calcium (Ca²⁺) – Present in lesser amounts compared to sodium and magnesium, usually around 3-4%.

4. Potassium (K⁺) – The least abundant of the four, contributing around 1% of the dissolved cations.

Why Other Options Are Incorrect:

• B. Mg > Na > Ca > K – This order is incorrect because sodium (Na) is more abundant than magnesium (Mg).

• C. K > Mg > Ca > Na – Potassium (K) is much less abundant than sodium (Na), so this order is wrong.

• D. Na > Mg > K > Ca – The positions of potassium (K) and calcium (Ca) are reversed in this option.

Conclusion:

The correct order of soluble cations found in saline water bodies is: Na > Mg > Ca > K.

Correct Answer: A